Anytime you attach a silencer to a barrel take a look down the bore to ensure it is aligned properly. A silencer misaligned enough to cause a baffle strike will be obviously canted to one side.
I have never heard of or seen shims used to align a silencer to a threaded barrel and I have seen more than a few. Get your barrel threaded properly and forget the shims. Can you post a photo of the barrel/threads?
Lark is free!